1. Field of the Invention
The present invention relates to an explosive grade ammonium nitrate product.
2. Description of the Background
Ammonium nitrate by itself is a very stable material and is incapable of exploding. However, when fuel oil such as #2 diesel oil is added to ammonium nitrate (AN) in an oxygen balanced ratio, the product produced (ANFO) is an effective blasting agent which requires a high explosive charge to initiate detonation. A high explosive, on the other hand, is a substance whose detonation can be initiated by moderate shock from the likes of a blasting cap. Actually, ANFO has long been known, as early as the 1860's. In order to be effective as a blasting agent the fuel oil must be distributed throughout the explosive grade ammonium nitrate. The simple coating of the surface of relatively large AN prills results in a poorly performing explosive product. In fact, it was the need for intimate ammonium nitrate-fuel oil contact that originally led to the development of low density explosive grade ammonium nitrate (Lo-D XAN), which is normally produced by prilling an ammonium nitrate melt containing a relatively high amount of moisture, usually about 4-8% by weight. As the molten AN droplets solidify in the prilling tower, the high moisture content of normally about 4-6% of the prills causes "sintering" of the prills or a prill structure containing many micropores. The prills are dried to a moisture content less than 0.5%, have a bulk density usually ranging from 0.72-0.8 gm/cm.sup.3 and have a Caking Index measured as high as about 4.4 kg/cm.sup.2 which is much greater than the prills of the present invention. Further, the prills have a moderate-to-low crushing strength of about 25 kg/cm.sup.2. Then the prills are coated with a conditioning agent or coating. The coating is necessary to reduce prill caking tendency and may to some extent, facilitate fuel oil absorption. However, this increased oil absorption is a surface effect, with none of the oil being dispersed into the interior of the prill where it is needed. In fact, in some instances the coating agent may plug surfacial entrances to the substrate pores which reduces the energy released upon detonation of the XAN.
The drying step in the production of low density explosive grade AN (Lo-D XAN) is very important, because several functions are served in this step. One aspect of drying is that drying promotes prill porosity and aids in the development of surface access to the internal micropore structure. The internal porosity of the prills is not of much value unless the structure provides access to the interior pores from the exterior. A second aspect is that drying eliminates excess prill moisture which can interfere with the subsequent absorption of oil resulting in a lessening of the explosive force generated by the AN-fuel oil (ANFO) composition. Yet another aspect is that drying removes moisture which leads to caking during storage of explosive-grade ammonium nitrate (XAN, which is processed into ANFO).
Most manufacturers of low density XAN do not form AN prills in the presence of a stabilizing additive because the additives have an adverse effect on the drying of the prills. As a consequence, XAN is not dimensionally stabilized, is inherently susceptible to prill breakdown, and cakes during storage. The problems caused by moisture retained in the prills and the substantial increased costs incurred by the necessity of having to use processing equipment capable of removing the relatively high amounts of moisture and the large amount of energy required to achieve drying are more significant factors than the advantages gained by use of internal additives. Still further, because of the low density of the ammonium nitrate, more blasting holes or larger diameter blasting holes must be drilled in order to obtain a sufficient blasting effect. Consequently, low density XAN manufacturers have to contend with inherent product quality problems.
In attempting to alleviate the caking problem of low density XAN, manufacturers use coating or "parting" agents in the preparation of the prilled product. However, only several of these coatings such as Petro Ag, Petro Ag-treated kaolin, and the like may be employed since many of the coating agents not only interfere with oil absorption by the prills, but they also diminish ANFO's explosive force. Thus, the degree of protection offered by the coating agent is usually much less than is desired.
Hurst, U.S. Pat. No. 4,093,478 discloses an explosive ammonium nitrate composition quite different from that of the present invention. The explosive of the reference is a two component formulation in which one component is a liquid fuel comprised of hydrocarbon derivatives having an oxygen equivalent weight less than about 4 grams per equivalent. The second component is activated ammonium nitrate prills apparently of the low density type. In order to increase the porosity of the prills, the same are treated with moisture in an amount ranging from 0.3 to 6% by weight which is subsequently evaporated to create voids therein. The ammonium nitrate however does not contain any internal additives and, because of its increased porosity, exhibits diminished stability, i.e., a markedly increased tendency to crumble and disintegrate. Further, this processing of ammonium nitrate is not the processing which occurs in the present invention.
Bachman et al, U.S. Pat. No. 4,736,683 describes an ammonium nitrate blasting material which is based upon a high density ammonium nitrate. In this invention, however, fuel oil retention is provided on the ammonium nitrate particles by a stringy, high molecular-weight polymer. The high density prills are not sufficiently porous to absorb fuel oil. Although Bachman et al teaches thermal cycling of prills to increase porosity, the increased porosity is achieved by cracking of the prill surfaces and probably the infrastructure of the prills. Such prills are structurally weakened unlike the XAN product produced by the process of the present invention.
Osako et al, U.S. Pat. No. 3,966,853 describes an explosive ammonium nitrate material prepared from ammonium nitrate prills in turn prepared by prilling ammonium nitrate containing from 2 to 7% water. The ammonium nitrate prills are thus low density ammonium nitrate prills. Accordingly, the patent does not show an explosive grade ammonium nitrate which is a high density material.
On the other hand, high density ammonium nitrate, because of its inherently low porosity, does not make a good AN-fuel oil, blasting agent. Unless the high density prills are made very small, the desired amount of fuel oil (albeit, on the surface of the AN prills) cannot be achieved. Normally, ANFO is such that the ammonium nitrate prills must absorb at least about 6% (wt.) fuel oil. High density AN prills which have a low moisture content, which is necessary for good storage characteristics, even when they contain an internal additive, do not make a very effective blasting agent.
It is further pointed out that ammonium nitrate production equipment is designed for the type of product it produces, i.e., high density AN or low density AN. Very little, if any, drying is needed for high density AN, and it can be prepared by simply cooling the ammonium nitrate, rather than by drying and then cooling ammonium nitrate as is required for low density AN. In the manufacture of high density AN, water is removed in the melt evaporator. High density AN evaporators, therefore, must have a greater water-removing capacity than low density AN evaporators at comparable production rates. It is possible (although not practical) to produce both low density and high density AN in a given production train, but this is only achieved at the expense of an extreme loss in production rate or a tremendous decline in product quality. These limitations greatly reduce the flexibility of AN processes. A need therefore continues to exist for a simplified way of producing explosive grade ammonium nitrate from high density fertilizer grade ammonium nitrate.